This invention relates to an information processing system operable according to a software program which defines instructions of a first and second kind.
In a manner known in the art, an information processing system is operable according to a microprogram which is memorized in a control storage and which defines a plurality of microinstructions. Each of the microinstructions is divided into a plurality of fields. The microinstructions are classified into horizontal microinstructions and vertical microinstructions. Each of the horizontal microinstructions is divided into many fields and consists of a large number of bits. On the other hand, each of the vertical microinstructions is divided into a few fields and consists of a small number of bits in comparison with those of the horizontal microinstructions. Therefore, the horizontal microinstructions are adopted in the information processing system rather than the vertical ones in order to raise processing performance in the information processing system. This is because the information processing system carries out several functions in one step or one machine cycle by adopting the microprogram which defines the horizontal microinstructions. More specifically, hardware of the information processing system is optimized so as to make the most of the characteristics of the horizontal microinstructions for instructions which are frequently used, such as basic operation instructions. Under the circumstances, the information processing system comprises a control storage unit, including a control storage implemented by a few IC memory chips.
However, sequential processing is carried out by the information processing system on many other instructions such as various control instructions, list processing instructions for repeatedly handling data in a main memory, and the like. When the information processing system carries out such sequential processing by adopting the microprogram which defines the horizontal microinstructions, a few fields are used in the fields of each horizontal microinstruction. This is because the information processing system only carries out one small function at one step, despite many other instructions. As a result, the control storage is hardly available in a word direction or in a longitudinal direction for each horizontal microinstruction.
In order to resolve such a defect, the information processing system realizes the other instructions such as the various control instructions by adopting an instruction program which defines the basic operation instructions realized by the microprogram. More specifically, the basic operation instructions are referred to as instructions of a first kind. The other instructions, such as the various control instructions, are called instructions of a second kind. In other words, each of the instructions of the first kind is realized by the microprogram. Each of the instructions of the second kind is realized by the instruction program. The information processing system is operable according to a software program which defines the instructions of the first kind and the instructions of the second kind.
Such an information processing system comprises a main memory for memorizing the instructions of the first kind and the instructions of the second kind. The main memory is connected to an instruction fetching unit for successively fetching, as a current instruction, each of the instructions of the first kind and the instructions of the second kind. The instruction fetching unit is connected to an instruction analyzing memory which acts as an instruction decoding arrangement for decoding the current instruction into a decoded signal. The instruction analyzing memory is connected to a control storage unit which serves as a microinstruction processing arrangement for successively processing the microinstructions to produce a processed signal in response to each of the microinstructions. The main memory, the instruction fetching unit, and the control storage unit are connected to an operand accessing unit for accessing the main memory in response to the processed signal to read an operand for the current instruction. The operand accessing unit, the instruction analyzing memory, and the control storage unit are connected to an executing unit for executing the current instruction on the operand in response to the decoded signal and the processed signal.
In a conventional information processing system, the operand accessing unit comprises a memory access mode register connected to the control storage unit for holding a memory access mode bit indicative of a memory access mode which defines an access mode for the main memory. The memory access mode bit is renewed by the processed signal. The memory access mode register, the instruction fetching unit, and the main memory are connected to an operand fetching unit for fetching the operand in accordance with the memory access mode indicated by the memory access mode bit. In the conventional information processing system, the contents of the memory access mode register must be read and stored in a specific register on starting the processing of the instruction program, in order to keep the state before the software program is branched to the instruction program. Then, the stored contents must be set in the memory access mode register on closing the processing of the instruction program. Therefore, the conventional information processing system is disadvantageous in that if suffers from more overhead during starting and closing processing of the instruction program.